Solid phase welding is a method of welding metals by the application of pressure so as to produce interfacial plastic deformation of the metals at the interfacial surfaces which breaks up the contaminant surface films to expose virgin contact surfaces for bonding.
A solid phase weld may be achieved by a process identified as "impact welding" which consists of driving or propelling one metal layer against another metal layer at a sufficient velocity and at an oblique impact so as to cause bonding of the two metal layers together at the common interfacial region of contact. Impact Welding has been achieved by those skilled in the art by utilizing magnetic propulsion equipment, gas guns and explosives to propel the metal layers together. If the metals are driven together by means of explosion, the process is known as explosion welding.
In explosion welding, metal plates or layers which are to be welded are spaced apart relative to one another in either generally parallel relation or inclined relation, and a layer of suitable explosive charge disposed on one of the metal layers is detonated so as to impart kinetic energy to the "flyer" plate causing the flyer plate to collide obliquely with the stationary "parent" plate. The explosive while detonating produces a force normal to the flyer plate causing the flyer plate to impact the parent plate obliquely at a collision or impact angle. As the detonation proceeds along the flyer plate, it progressively drives the flyer plate along the parent plate at a particular welding velocity. If two metal layers are to be bonded the explosive charge may be disposed on both metal layers.
U.S. Pat. Nos. 3,728,780 and 3,137,937 generally relate to explosion welding, which may be utilized to weld different metals together.
U.S. Pat. No. 3,813,758 teaches that a metal jet is formed at the point of impact between the flyer plate and parent plate. It is believed that this jet which contains the contaminant surface layers of both plates is forced outwardly at a high velocity during the explosion welding process. This cleaning operation allows a solid phase weld to be formed between the interfacial virginally clean metallic surfaces of the plates under the intense local pressure in the region of contact.
U.S. Pat. No. 3,583,062 discloses that three types of bonded zones may result from explosion welding, namely:
(a) a direct metal to metal bond (with a straight interface); PA1 (b) a uniform melted layer in which the metals are bonded together with an intervening layer of solidified melt of substantially homogeneous composition; PA1 (c) a wavy type of bond zone comprised of periodically spaced discreet regions of solidified melt, between areas of direct metal to metal bond.
Moreover, U.S. Pat. No. 3,397,444 generally teaches that products having the wavy type bond interface are preferred in many situations because of their normally higher strength, and defines values of parameters such as collision velocity so as to produce the preferred wavy interface.
Similarly, U.S. Pat. No. 3,583,062 states that the wavy bond zone is preferred over the substantially straight bond because of the larger interfacial area the wavy bond provides, and also defines the value of certain parameters which will produce the preferred wavy interface.
However for metal combinations tending to form brittle intermetallics, the melt associated with the bonded wavy interface presents zones of weakness. Metal combinations which tend to form brittle combinations are well known to those skilled in the art and generally encompass those metal combinations which have a wide dissimilarity between the densities of the metals to be bonded, which include for example, aluminum to steel, zirconium to steel , tantalum to steel, titanium to steel, titanium to copper, and their respective alloys.
Brittle intermetallics are diffusion products, and are undesirable, particularly when the welded zone is subjected to an increase in temperature which enhances diffusion.
Diffusion may be defined as a transfer of atoms into the vacancies and interstitial spaces from one metal to another; and diffusion is enhanced with an increase in temperature in the region of interfacial contact. In particular diffusion is enhanced in the region of pockets of melt associated with the bonded wavy interface as, during the welding process, these regions are subjected to elevated temperatures, due to the adiabatic rise of same at the vortex of each wave. Moreover, for the wavy morphology to occur, the entire interface has to be subjected to a higher energy than that necessary for a straight interface which will consequently produce larger plastic deformation and hence higher temperatures, further enhancing diffusion.
Furthermore, mechanical solicitation (such as dynamic or static bending) applied to a wavy interface causes the weld to fail in metal combinations capable of forming brittle intermetallics, as the interfacial pockets of melt create zones of weakness.
Efforts have been made to retard the diffusion process in the bonded zone particularly for those metal combinations capable of forming brittle intermetallics and particularly when such metal combinations are exposed to an elevated temperature, by utilizing diffusion barriers. In the case of aluminum to steel such barriers are titanium, nickel, chromium, molibdenum, silver, etc., or ferritic stainless steel as disclosed in Canadian Pat. No. 917,869, which are sandwiched between and metallurgically bonded between the flyer and parent plate. However, the use of such barriers increases the cost of the explosion welded product, and their efficiency is quite relative.